John Homenuk

Myriad of threats expected from midweek storm

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[alert type=”error”]Storm Prediction Center has placed parts of the area in a Moderate Risk for severe weather on Thursday. Details. [/alert]

A strong storm system traversing the Ohio Valley will strengthen and shift towards the Northeast US on Thursday, bringing the potential for several types of hazardous weather. Forecast models suggest the system may be anomalously strong for this time of year, with pressures dropping to near 990mb as it moves through the Mid Atlantic states. This is absolutely unprecedented for this time of year, as normally the jet stream is too weak this time of year to support such dynamic systems.

A warm front is forecast to lift northward from the Mid Atlantic states to a position near Central New Jersey on Thursday afternoon. The combination of building instability near and south of the warm front, as well as favorable wind fields and forcing for precipitation will lead to the potential for not only heavy rain and strong winds, but severe weather…including the potential for a few tornadoes.

If you have not already, I highly recommend you check out our previous technical discussion, as it will help to explain some concepts that I will talk about in detail for the rest of this post, and give you a pretext as to what is going on.

The exact position and timing of the surface low, cold front, and warm front will be crucial in determining where the tornado risk is, and where the northern boundary of the severe weather can be expected. Latitudes north of the surface low can expect heavy rain of 2-4″ and little in the way of severe weather, areas with the same latitude as the surface low can expect 1-2″ of rain with a chance of being hit by a squall line producing borderline severe winds, and latitudes south of the surface low can expect severe weather and a few tornadoes. The problem is, the exact position of the surface low is still a bit uncertain, but we’ll do our best to warn everyone of the possibilities.

The 00z 6/12 NAM valid for 2 p.m. on Thursday.

The 00z 6/12 NAM valid for 2 p.m. on Thursday.

Our first two images will illustrate the NAM’s boundaries and its movements throughout the day on Thursday. The NAM, at 2 p.m. on Thursday is forecasting a 990mb surface low in SW PA (top left panel). You can see two distinct temperature gradients — one to the south of the low, and one to the east of the low. I quickly highlighted the approximate warm front position in red and the cold front position in blue, at that time.

As I said in yesterday’s discussion, the cold front often triggers a linear convective mode. But look at how far east the warm front extends away from the front! Areas just south of the warm front and away from the cold front can expect a discrete supercellular convective mode. Thus, at 2 p.m., SE PA, SW NJ, DE, and MD could really be under the gun for a tornado threat, and perhaps severe hail as well.

A very key fact that one must understand for this threat: the slower the cold front advances, the longer the warm front will be the main boundary for thunderstorm initiation in the very favorable tornadic environment, thus discrete supercells. A faster moving cold front means that it would eventually be the dominating player in the favorable tornadic environment, potentially “wasting” the environment and turning the convective mode more linear, since it would more quickly intercept that environment. Linear squall lines can still provide embedded tornadoes, so a squall line does not mean there is no tornado threat, but it would be significantly reduced compared to that of more discrete supercells.

Also note the southeast surface winds that even exist to the south of the warm front in SE PA on the bottom left panel — when you combine that with the potent mid level winds from the WSW associated with this powerful storm, you get strong, deep-layer shear supportive for severe weather and even tornadoes. More will be discussed about the wind profiles later in this post.

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Doug Simonian

Severe Storms Likely for the Mid-Atlantic on Thursday. How Far North Will They Go?

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As well evidenced by the recent inclement weather, the northeast is under a very active weather pattern, and after a couple of days of general relaxation, the weather looks to turn unsettled again on Thursday.

A potent storm system with lots of vorticity will be the culprit for severe weather on Thursday.

A potent storm system with lots of vorticity will be the culprit for severe weather on Thursday.

A powerful storm system will quickly traverse the country, giving the Midwest and Ohio Valley a severe weather event on Wednesday. This will head eastward and approach our area for Thursday. At this point, all eyes will turn towards the northern Mid-Atlantic states: in particular, southeastern Pennsylvania, Maryland, Delaware, northeast Virginia, and perhaps central and southern New Jersey as well. This storm system will provide very strong winds — well in excess of 50 knots — at 500mb, coming out of the WSW; generally a very conducive environment for severe weather. In addition, the potent vorticity, among other factors that will be explained shortly, will also be conducive for lift, in order to generate thunderstorms.

What is often true about these powerful storm systems is that they possess strong boundaries, such as cold fronts and warm fronts. It is along these boundaries, and on the warm sides of them, where we pay particularly close attention, because the warm, moist air in the warm sector is lifted near the boundaries themselves, helping to initiate convection, and thus, thunderstorms. Usually, a cold front in itself is a boundary that is only conducive for squall lines, because they are expansive, they favor lots of lift over the entire front (as opposed to a dry-line, where the lift is more localized), and move quickly — so the best lift continually catches up with any storms that form, causing more and more storms to form in close proximity to them, thus quickly favoring congealing into a line. This is what is typical for northeast thunderstorm setups, and one reason why you tend to see large lines of storms, rather than the discrete supercells that you get in the Plains. However, in addition to this cold front, there will also be a warm front extending from west to east, well out ahead of the cold front.

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El Reno, OK tornado becomes widest ever

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The National Weather Service has confirmed, after a damage survey, that the tornado which struck El Reno, Oklahoma on May 31st was an EF5 with radar measured winds of 296 miles per hour. Most notably, the tornado reached a maximum width of 2.6 miles, making it the widest tornado ever measured on earth. The wind speeds nearly set a record as well, falling just shy of the strongest winds recorded in a tornado (301 mph, Moore OK tornado in 1999 still maintains the record).

Initially rated an EF-3 on the new Enhanced Fujita Scale, which rates tornadoes from EF0 to EF5, the tornado was upgraded after mobile doppler radar data showed the intense wind speeds of near 300 miles per hour. The winds were measured on mobile doppler radars from two graduate students traveling with the University of Oklahoma. Tornado researcher Howard Bluestein, a professor at the University of Oklahoma put it simply when he said “This is the biggest ever” of the tornado.

Animation of radar data, captured every minute, from Phased Array Radar. Courtesy of Robin Tanamachi.

Four storm chasers were killed in the same tornado, which will certainly become one of the most historic ever for a multitude of reasons. Tim Samaras, Paul Samaras, Carl Young, and Charles Henderson were killed while chasing the storm. Recently, high resolution radar imagery reveals an incredibly impressive structure while the tornado was on the ground, and a debris ball indicated — which takes a dramatic, sharp and sudden turn to the north to a point near where the four chasers were positioned.

For more information on the El Reno, Oklahoma tornado we suggest visiting the National Weather Service in Norman, Oklahoma as well as other local news sources.

Doug Simonian

Medium Range Snippet – Potential Severe Weather on May 22nd?

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The 12z Euro from May 13 shows a potent trough in the Eastern US, potentially favoring severe weather for the Mid Atlantic and Northeast.

The 12z Euro from May 13 shows a potent trough in the Eastern US, potentially favoring severe weather for the Mid Atlantic and Northeast on May 21 or 22.

 

I’m sure a lot of you have been wondering where all of the tornadoes been in tornado alley this year. Additionally, some of you are probably wondering when the first widespread severe weather outbreak will occur in the Northeast. I’ll try to give a quick discussion on that in this post.

The problem for tornadoes in tornado alley this year (the Southern Great Plains) is that there has been so much blocking and a slower Pacific Pattern, allowing disturbances to slow down as they traverse the country and become monster cutoff lows. Cutoff lows can often be good for severe weather, but when they are too large and too far east, they cut off the moisture and heat supply from the Gulf of Mexico. This is why there has been record breaking cold in the Plains, and also partially why our weather has been pretty chilly thus far.

However, this is expected to change in the next few days. The MJO (Madden Julian Oscillation) is expected to change into phases 4 and 5, and then move into “the circle of death”, where it will stop having an effect. This leads to a much more active Pacific Jet pattern, a bit more troughing on the west coast, and an attempt to raise the heights on the east coast (from moving into phases 4 and 5). The MJO going into the circle of death prevents the MJO from moving into the phases that do not favor troughing and an active Pacific Jet. This prevents the phase 4 and 5 pattern from changing.

This all will lead to some moisture return and heat from the Gulf of Mexico reaching the Plains and our area as well. The active Pacific Jet will lead to storm systems of decent strength, and combine that with good heat and moisture return from the Gulf, that “clash” can lead to severe storms.

Starting on Friday and through the weekend, storms should start to fire — perhaps significantly so — in the High Plains regions, and the severe storm threats will gradually shift eastward as the trough/storm system moves east. By around the May 21 or May 22 time period, the base of the trough may very well be over the southern Ohio Valley. If the trough is potent enough, it will lead to strong mid and upper level winds in our area, along with low level veering in the boundary layer from the return flow from a developing surface low.

I don’t want to sound a “hype” alarm, as many things can still change. But things are looking pretty good for a decent round of storms for the Plains during the upcoming weekend, and perhaps a decent round of storms for the Mid Atlantic and Northeast on May 21 or May 22. It’s not as easy to get severe weather in our area as it is in the Plains, but this should still be something to keep in mind as we move forward.